In portable battery operated products such as a radio frequency communication device, it is desirable to have the lowest possible overall current drain in order to maximize battery life. Moreover, it is desirable to operate such products at the lowest possible voltage so as to minimize their total power consumption.
Conventional radio frequency communication devices may use one or more application specific integrated circuits to implement functions such as phase lock loops to synthesize frequencies needed for digital logic or radio frequency circuits. To conserve power, a synthesizer or other circuitry implemented in an application specific integrated circuit should be operated using as low a voltage as possible. Moreover, to conserve even more power, these circuits may be operated in a power saving mode where one or more of the circuits are switched on during active processing periods (e.g., signal transmission or reception, data storage, retrieval, or presentation) and off during "sleep"or "rest"periods. Operating in this fashion, a portable battery operated product can substantially increase available battery life, thus resulting in more usable "talk time"in a radio frequency communication device such as a cellular telephone or the like.
In the past, low voltage circuitry implemented in application specific integrated circuits typically consisted of bipolar analog or I.sup.2 L (integrated injection logic) logic circuits. These bipolar circuits experienced problems such as poor high speed operation (I.sup.2 L operating at 0.25 .mu.A per gate is typically operational to only around 50 KHz), a lack of dynamic range (conventional low bipolar analog circuits have a saturation point of typically 200 mY, yielding a range of less than 600 mY from a one volt supply), and extreme variation of their intrinsic operating characteristics over temperature.
Thus, what is needed is low voltage CMOS (complimentary metal oxide semiconductor) process and appropriate circuit topologies that allow a designer to achieve both analog and digital functions using an application specific integrated circuit in a radio frequency communication device. As such, the low voltage CMOS designs would operate at significantly lower power levels than comparable bipolar designs. Moreover, when operated in a power saving mode, the CMOS designs can more effectively conserve power while offering improved circuit performance characteristics.